Vitiligo is a cutaneous disease in which there is a complete loss of pigment in localized areas of the skin. This loss of pigment results in the effected areas being completely white. This condition has a predilection for the skin around the mouth and eyes. The result is cosmetically disfiguring, especially for dark skinned people. Furthermore, the depigmented skin is sun sensitive, and thus is subject to sunburns and skin cancer. In sum, vitiligo is both cosmetically and practically distressing to patients afflicted with the disease.
In normal skin, varying shades of brown are seen (depending on a person's race) representing the pigment melanin. This pigment is produced by a cell type known as a melanocyte. In vitiligo, there is an absence of melanocytes in the areas afflicted with the disorder. An absence of melanocytes results in an absence of melanin pigment, and thus the melanin-free area is white. Normal skin responds to ultraviolet light with an increase in the brown pigment melanin (tanning). Specifically, ultraviolet radiation stimulates melanocytes to proliferate and produce more melanin.
Attempts have also been made to “tan” vitiligo areas using ultraviolet light treatments. The ultraviolet spectrum is divided into two portions, “UVA” and “UVB,” which is light of 320-400 nm and 290-320 nm in wavelength, respectively. UVB is much more effective at producing a tan in normal skin. In normal skin, melanocytes reside in the epidermis, which is the outer layer of the skin. The epidermis is only 0.1 mm thick, so the melanocytes are very near the surface. UVB radiation can only penetrate to about 0.1 mm, but this is sufficient to reach the melanocytes. In patients with vitiligo, these epidermal melanocytes are gone. In some cases, there are surviving melanocytes deeper in the skin down the hair follicles. These melanocytes may be several millimeters deep. UVB cannot penetrate this deep in the skin to stimulate these surviving deep melanocytes. Exposure to UVB results in a sunburn at the surface of the skin with no stimulation of these deep melanocytes. Thus attempts to repopulate the vitiligo areas with melanocytes deep in the skin in response to UVB exposure have failed. UVA will penetrate a bit deeper in the skin than UVB. However, UVA is very poor at stimulating melanocytes to proliferate and migrate.